1. Field of the Invention
The present invention relates to a method and device for controlling the driving of an exhaust valve in an internal combustion engine, and more particularly to an exhaust valve drive control method and device in which a closing operation of the exhaust valve can be modeled easily such that the exhaust valve is controlled to close on the basis of this model.
2. Description of the Related Art
In recent years, valve mechanisms which drive a valve to open and close using fluid pressure rather than a cam mechanism have been proposed (see Japanese Unexamined Patent Application Publication 2003-328713 and Japanese Unexamined Patent Application Publication 2001-280109, for example) in order to enhance the freedom with which an exhaust valve and intake valve of an internal combustion engine (“engine” hereafter) are controlled to open and close. When such a valve mechanism is used, the open/close timing, displacement, and so on of the exhaust valve and intake valve can be adjusted and controlled in accordance with the operating conditions of the engine, thereby enabling finer engine control. Particularly when the exhaust valve is controlled to close, scavenging can be performed reliably and effectively while avoiding contact between the exhaust valve and a rising piston.
For example, the present applicant proposes valve closing control such as that shown in FIG. 7.
A line A in the drawing denotes the displacement of the exhaust valve (the position of the lower end of the exhaust valve), and a line B denotes a piston position (the position of the upper end of the piston). The lower end of the ordinate shows the position of the exhaust valve when fully closed (displacement zero). Moving upward steadily along the ordinate, the displacement (opening) of the exhaust valve increases and the piston position falls. In other words, in FIG. 7 the positional relationship and traveling direction of the exhaust valve and the piston are illustrated upside-down from the actual positional relationship and traveling direction.
FIG. 7A shows an example in which the engine rotation speed is comparatively low, and the piston rising speed is lower than the exhaust valve closing speed (rising speed).
As shown in the drawing, the exhaust valve closing operation begins before the piston rises to the lower end of the fully open exhaust valve. Here, the rising speed of the piston is lower than the closing speed of the exhaust valve, and hence when the exhaust valve closing operation begins, the gap between the piston and the exhaust valve increases gradually. Once the gap between the piston and exhaust valve has increased to a predetermined value, the exhaust valve closing operation is halted temporarily. Then, when the gap between the rising piston and the exhaust valve closes to a certain extent, the closing operation is resumed. In other words, the exhaust valve closing operation is executed in stages according to the rising of the piston. By closing the exhaust valve in stages in this manner, a sufficient opening area can be secured for the exhaust outlet, and hence the scavenging efficiency can be improved.
However, the rising speed of the piston naturally varies according to the engine rotation speed, and therefore the content of the exhaust valve closing control must be modified for every engine rotation speed.
For example, in a region in which the piston rising speed is greater than the exhaust valve closing speed, the piston may contact the exhaust valve if the exhaust valve is closed in stages, and hence, as shown in FIG. 7B, the exhaust valve must be closed continuously (in one operation) from the fully open position to the fully closed position. In this case, the exhaust valve is driven once, and the driving period is comparatively long. Note that the valve closing control shown in FIG. 7 was an unpublished technique at the time of the filing (Jun. 17, 2004) of the Japanese Patent Application from which this application claims priority, and does not constitute prior art.
Since the content of the optimum closing control for the exhaust valve (the number of times the valve is driven, the driving timing, the driving period, and so on) differs according to the engine rotation speed in this manner, control maps defining the optimum control content for each engine rotation speed are created conventionally, but since a large number of control maps is required, an extremely large amount of labor is involved in creating the maps.